1. Field of the Invention
The present invention relates to photoconductive toners, more specifically, to photoconductive toners which are sensitive to red light, green light or blue light, and to photoconductive toners possessing photosensitivity in the laser wavelength region.
2. Description of the Art
Recently, considerable attention has been directed toward methods for the formation of colored images with just a single exposure by using mixed toners consisting of three varieties of photoconductive toners which have been colored cyan, magenta and yellow, respectively (i.e., a cyan toner, a magenta toner, and a yellow toner).
Each of these colored toners possesses sensitivity to light of the complementary color, i.e., the cyan toner is sensitive to red light, the magenta toner is sensitive to green light, and the yellow toner is sensitive to blue light. The respective photoconductive toners acquire photoconductivity by exposure to the corresponding colors of light. Therefore, for example, if a mixed toner is compounded by mixing a yellow toner which manifests photosensitivity with respect to light in the vicinity of 450 nm, a magenta toner which manifests photosensitivity with respect to light in the vicinity of 550 nm, and a cyan toner which manifests photosensitivity with respect to light in the vicinity of 650 nm, then, if this mixed toner is used in a one-shot color system, a colored image can be formed by a single exposure.
However, in order to prevent an undesirable mixture of colors, the photosensitive wavelength regions of the three colored photoconductive toners employed in such a one-shot color system must be separated, thus, three varieties of photoconductive toners with photosensitivity in the vicinity of 450 nm, 550 nm, and 650 nm, respectively, as indicated above, are regarded as necessary for this purpose.
Such photoconductive toners ordinarily contain a resin binder, zinc oxide, and a dye sensitizer; the present applicant has previously filed on application relating to a photoconductive toner employing cyanine dyes as the dye sensitizers (i.e., Japanese Patent Application Nos. 1-150935, 1-300365, and 1-300366). However, when this type of cyanine dye was used as the dye sensitizer, the photosensitivity of the toner dropped in some cases where a relatively large quantity of the dye was added. Therefore, a photoconductive toner satisfying both the requirements of displaying the necessary hue and possessing sufficient sensitivity was difficult to obtain, and consequently vivid images could not be formed with this toner.
In particular, cyanine dyes have the disadvantage in that the sensitivity of such dyes in the vicinity of 450 nm is markedly low as compared with that of dye sensitizers with sensitivity in other wavelength regions. Moreover, if a photoconductive toner is prepared using dye sensitizers other than the aforesaid cyanine dyes, then another disadvantage arises in that, for example, if fluorescein is used as the sensitizer dye for blue light, then, although the sensitizing effect upon zinc oxide is comparatively great, the sensitive wavelength is shifted toward the long wavelength side, resulting in a poor hue of the toner.
On the other hand, laser printers have come into wide use in recent years, so there now exists a need for photoconductive toners with photosensitivity in the near infrared to infrared region.